Ohmic N-contact formed at low temperature in inverted metamorphic multijunction solar cells

ABSTRACT

A method of forming a multijunction solar cell including an upper subcell, a middle subcell, and a lower subcell by providing a substrate for the epitaxial growth of semiconductor material; forming a first solar subcell on the substrate having a first band gap; forming a second solar subcell over the first solar subcell having a second band gap smaller than the first band gap; forming a graded interlayer over the second subcell, the graded interlayer having a third band gap greater than the second band gap; forming a third solar subcell over the graded interlayer having a fourth band gap smaller than the second band gap such that the third subcell is lattice mismatched with respect to the second subcell; and forming a contact composed of a sequence of layers over the first subcell at a temperature of 280° C. or less and having a contact resistance of less than 5×10 −4  ohms-cm 2 .

This is a continuation of patent application Ser. No. 14/284,909, filedMay 22, 2014, which is a continuation of U.S. patent application Ser.No. 13/603,088, filed Sep. 4, 2012, now U.S. Pat. No. 8,753,918, whichis a divisional of U.S. patent application Ser. No. 12/218,582, filedJul. 16, 2008, all of which are herein incorporated by reference intheir entireties.

REFERENCE TO RELATED APPLICATIONS

This application is related to co-pending U.S. patent application Ser.No. 12/123,864 filed May 20, 2008.

This application is related to co-pending U.S. patent application Ser.No. 12/102,550 filed Apr. 14, 2008.

This application is related to co-pending U.S. patent application Ser.No. 12/047,842, and U.S. Ser. No. 12/047,944, filed Mar. 13, 2008.

This application is related to co-pending U.S. patent application Ser.No. 12/023,772, filed Jan. 31, 2008.

This application is related to co-pending U.S. patent application Ser.No. 11/956,069, filed Dec. 13, 2007.

This application is also related to co-pending U.S. patent applicationSer. Nos. 11/860,142 and 11/860,183 filed Sep. 24, 2007.

This application is also related to co-pending U.S. patent applicationSer. No. 11/836,402 filed Aug. 8, 2007.

This application is also related to co-pending U.S. patent applicationSer. No. 11/616,596 filed Dec. 27, 2006.

This application is also related to co-pending U.S. patent applicationSer. No. 11/614,332 filed Dec. 21, 2006.

This application is also related to co-pending U.S. patent applicationSer. No. 11/445,793 filed Jun. 2, 2006.

This application is also related to co-pending U.S. patent applicationSer. No. 11/500,053 filed Aug. 7, 2006.

GOVERNMENT RIGHTS STATEMENT

This invention was made with government support under Contract No.FA9453-06-C-0345 awarded by the U.S. Air Force. The Government hascertain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of semiconductor devices, andto processes requiring the formation of ohmic contacts at relatively lowtemperatures. More particularly, the invention relates to fabricationprocesses and devices such as multijunction solar cells based on III-Vsemiconductor compounds including a metamorphic layer. Such devices arealso known as inverted metamorphic multijunction solar cells.

2. Description of the Related Art

Photovoltaic cells, also called solar cells, are one of the mostimportant new energy sources that have become available in the pastseveral years. Considerable effort has gone into solar cell development.As a result, solar cells are currently being used in a number ofcommercial and consumer-oriented applications. While significantprogress has been made in this area, the requirement for solar cells tomeet the needs of more sophisticated applications has not kept pace withdemand. Applications such as concentrator terrestrial power systems andsatellites used in data communications have dramatically increased thedemand for solar cells with improved power and energy conversioncharacteristics.

In satellite and other space related applications, the size, mass andcost of a satellite power system are dependent on the power and energyconversion efficiency of the solar cells used. Putting it another way,the size of the payload and the availability of on-board services areproportional to the amount of power provided. Thus, as the payloadsbecome more sophisticated, solar cells, which act as the powerconversion devices for the on-board power systems, become increasinglymore important.

Solar cells are often fabricated in vertical, multijunction structures,and disposed in horizontal arrays, with the individual solar cellsconnected together in a series. The shape and structure of an array, aswell as the number of cells it contains, are determined in part by thedesired output voltage and current.

Inverted metamorphic solar cell structures such as described in M. W.Wanlass et al., Lattice Mismatched Approaches for High Performance,III-V Photovoltaic Energy Converters (Conference Proceedings of the31^(st) IEEE Photovoltaic Specialists Conference, Jan. 3-7, 2005, IEEEPress, 2005) present an important conceptual starting point for thedevelopment of future commercial high efficiency solar cells. Thestructures described in such reference present a number of practicaldifficulties relating to the appropriate choice of materials andfabrication steps, for a number of different layers of the cell.

Prior to the present invention, the materials and fabrication stepsdisclosed in the prior art have not been adequate to produce acommercially viable and energy efficient solar cell using commerciallyestablished fabrication processes for producing an inverted metamorphicmultijunction cell structure.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the present invention provides a methodof forming a multijunction solar cell comprising an upper subcell, amiddle subcell, and a lower subcell, the method comprising; providing afirst substrate for the epitaxial growth of semiconductor material;forming an upper first solar subcell on said first substrate having afirst band gap; forming a middle second solar subcell over said firstsolar subcell having a second band gap smaller than said first band gap;forming a graded interlayer over said second solar cell; forming a lowerthird solar subcell over said graded interlayer having a fourth band gapsmaller than said second band gap such that said third subcell islattice mismatched with respect to said second subcell; and forming acontact having a contact resistance of less than 2×10⁻⁴ ohms-cm² oversaid first subcell at a temperature of 210° C. or less.

In another aspect, the present invention provides a method ofmanufacturing a solar cell including; providing a first semiconductorsubstrate for the epitaxial growth of semiconductor material; forming afirst subcell on said substrate comprising a first semiconductormaterial with a first band gap and a first lattice constant; forming asecond subcell comprising a second semiconductor material with a secondband gap and a second lattice constant, wherein the second band gap isless than the first band gap and the second lattice constant is greaterthan the first lattice constant; and forming a lattice constanttransition material positioned between the first subcell and the secondsubcell, said lattice constant transition material having a latticeconstant that changes gradually from the first lattice constant to thesecond lattice constant; and forming a contact at a temperature of 280°C. or less, and having a contact resistance of less than 5×10⁻⁴ohms-cm².

In still another aspect, the present invention provides a method ofmanufacturing a solar cell including; providing a first semiconductorsubstrate; depositing on a first substrate a sequence of layers ofsemiconductor material forming a solar cell forming a contact having aresistance of less than 5×10⁻⁴ ohms-cm²; mounting a surrogate secondsubstrate on top of the sequence of layers; and removing the firstsubstrate.

In still another aspect, the present invention provides, more generally,a method of forming a semiconductor device including providing asubstrate for the epitaxial growth of semiconductor material; formingfirst active layer on said substrate; forming a second active layer oversaid first active layer; and forming an electrical contact having acontact resistance of less than 2×10⁻⁴ ohms-cm² over at least one ofsaid active layers at a temperature of 210° C. or less.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better and more fully appreciated by reference tothe following detailed description when considered in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a graph representing the bandgap of certain binary materialsand their lattice constants;

FIG. 2 is a cross-sectional view of the solar cell of the inventionafter the deposition of semiconductor layers on the growth substrate;

FIG. 3 is a cross-sectional view of the solar cell of FIG. 2 after thenext process step;

FIG. 4 is a cross-sectional view of the solar cell of FIG. 3 after nextprocess step;

FIG. 5A is a cross-sectional view of the solar cell of FIG. 4 after thenext process step in which a surrogate substrate is attached;

FIG. 5B is a cross-sectional view of the solar cell of FIG. 5A after thenext process step in which the original substrate is removed;

FIG. 5C is another cross-sectional view of the solar cell of FIG. 5Bwith the surrogate substrate on the bottom of the Figure;

FIG. 6 is a simplified cross-sectional view of the solar cell of FIG. 5Cafter the next process step;

FIG. 7 is a cross-sectional view of the solar cell of FIG. 6 after thenext process step;

FIG. 8 is a cross-sectional view of the solar cell of FIG. 7 after thenext process step;

FIG. 9 is a cross-sectional view of the solar cell of FIG. 8 after thenext process step;

FIG. 10A is a top plan view of a wafer in which the solar cells arefabricated;

FIG. 10B is a bottom plan view of a wafer in which the solar cells arefabricated;

FIG. 11 is a cross-sectional view of the solar cell of FIG. 9 after thenext process step;

FIG. 12 is a cross-sectional view of the solar cell of FIG. 11 after thenext process step;

FIG. 13 is a top plan view of the wafer of FIG. 12 depicting the surfaceview of the trench etched around the cell;

FIG. 14A is a cross-sectional view of the solar cell of FIG. 12 afterthe next process step in a first embodiment of the present invention;

FIG. 14B is a cross-sectional view of the solar cell of FIG. 14A afterthe next process step in a second embodiment of the present invention;

FIG. 15 is a cross-sectional view of the solar cell of FIG. 14B afterthe next process step in a third embodiment of the present invention;

FIG. 16 is a graph of the doping profile in a base layer in themetamorphic solar cell according to the present invention; and

FIG. 17 is a graph that depicts the specific contact resistance ofcontact structures according to the present invention using thetransmission line method.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Details of the present invention will now be described includingexemplary aspects and embodiments thereof. Referring to the drawings andthe following description, like reference numbers are used to identifylike or functionally similar elements, and are intended to illustratemajor features of exemplary embodiments in a highly simplifieddiagrammatic manner. Moreover, the drawings are not intended to depictevery feature of the actual embodiment nor the relative dimensions ofthe depicted elements, and are not drawn to scale.

The basic concept of fabricating an inverted metamorphic multijunction(IMM) solar cell is to grow the subcells of the solar cell on asubstrate in a “reverse” sequence. That is, the high band gap subcells(i.e. subcells with band gaps in the range of 1.8 to 2.1 eV), whichwould normally be the “top” subcells facing the solar radiation, aregrown epitaxially on a semiconductor growth substrate, such as forexample GaAs or Ge, and such subcells are therefore lattice-matched tosuch substrate. One or more lower band gap middle subcells (i.e. withband gaps in the range of 1.2 to 1.8 eV) can then be grown on the highband gap subcells.

At least one lower subcell is formed over the middle subcell such thatthe at least one lower subcell is substantially lattice-mismatched withrespect to the growth substrate and such that the at least one lowersubcell has a third lower band gap (i.e. a band gap in the range of 0.7to 1.2 eV). A surrogate substrate or support structure is provided overthe “bottom” or substantially lattice-mismatched lower subcell, and thegrowth semiconductor substrate is subsequently removed. (The growthsubstrate may then subsequently be re-used for the growth of a secondand subsequent solar cells).

The present invention is directed to the composition of the metalcontact used for the grid lines and bus bar on the top (sunward facing)side of the solar cell. As noted above, one aspect of fabrication of anIMM solar cell is the requirement for attachment to a surrogatesubstrate of support (also called a “handler”) during fabrication. Suchattachment is typically done by a temporary adhesive.

The commercially available temporary adhesives have a relatively lowmelting point of around 100° C., and maintain adhesion to somewhatgreater than 200° C. These relatively low operating temperatures placecritical restrictions on the alloy temperature needed for forming anohmic metal contact to the semiconductor layers of the cell, especiallyto an n-type GaAs layer with a Au—Ge eutectic based alloy with eutectictemperature of 361° C. Normal alloying temperature is typically greaterthan 360° C. The current commercial production triple junction solarcells use a 365° C. temperature to alloy simultaneously a Ti/Au/Ag toboth the n and p-contact. From an electric standpoint, these contactsare rather poor, with specific contact resistivity of greater than1×10⁻³ greater than Ω-cm².

FIG. 1 is a graph representing the band gap of certain binary materialsand their lattice constants. The band gap and lattice constants ofternary materials are located on the lines drawn between typicalassociated binary materials (such as the ternary material GaAlAs beinglocated between the GaAs and AlAs points on the graph, with the band gapof the ternary material lying between 1.42 eV for GaAs and 2.16 eV forAlAs depending upon the relative amount of the individual constituents).Thus, depending upon the desired band gap, the material constituents ofternary materials can be appropriately selected for growth.

The lattice constants and electrical properties of the layers in thesemiconductor structure are preferably controlled by specification ofappropriate reactor growth temperatures and times, and by use ofappropriate chemical composition and dopants. The use of a vapordeposition method, such as Organo Metallic Vapor Phase Epitaxy (OMVPE),Metal Organic Chemical Vapor Deposition (MOCVD), Molecular Beam Epitaxy(MBE), or other vapor deposition methods for the reverse growth mayenable the layers in the monolithic semiconductor structure forming thecell to be grown with the required thickness, elemental composition,dopant concentration and grading and conductivity type.

FIG. 2 depicts the multijunction solar cell according to the presentinvention after the sequential formation of the three subcells A, B andC on a GaAs growth substrate. More particularly, there is shown asubstrate 101, which is preferably gallium arsenide (GaAs), but may alsobe germanium (Ge) or other suitable material. For GaAs, the substrate ispreferably a 15° off-cut substrate, that is to say, its surface isorientated 15° off the (100) plane towards the (111)A plane, as morefully described in U.S. patent application Ser. No. 12/047,944, filedMar. 13, 2008.

In the case of a Ge substrate, a nucleation layer (not shown) isdeposited directly on the substrate 101. On the substrate, or over thenucleation layer (in the case of a Ge substrate), a buffer layer 102 andan etch stop layer 103 are further deposited. In the case of GaAssubstrate, the buffer layer 102 is preferably GaAs. In the case of Gesubstrate, the buffer layer 102 is preferably InGaAs. A contact layer104 of GaAs is then deposited on layer 103, and a window layer 105 ofAlInP is deposited on the contact layer. The subcell A, consisting of ann+ emitter layer 106 and a p-type base layer 107, is then epitaxiallydeposited on the window layer 105. The subcell A is generally latticedmatched to the growth substrate 101.

It should be noted that the multijunction solar cell structure could beformed by any suitable combination of group III to V elements listed inthe periodic table subject to lattice constant and bandgap requirements,wherein the group III includes boron (B), aluminum (Al), gallium (Ga),indium (In), and thallium (T). The group IV includes carbon (C), silicon(Si), germanium (Ge), and tin (Sn). The group V includes nitrogen (N),phosphorous (P), arsenic (As), antimony (Sb), and bismuth (Bi).

In the preferred embodiment, the emitter layer 106 is composed ofInGa(Al)P and the base layer 107 is composed of InGa(Al)P. The aluminumor Al term in parenthesis in the preceding formula means that Al is anoptional constituent, and in this instance may be used in an amountranging from 0% to 30%. The doping profile of the emitter and baselayers 106 and 107 according to the present invention will be discussedin conjunction with FIG. 16.

Subcell A will ultimately become the “top” subcell of the invertedmetamorphic structure after completion of the process steps according tothe present invention to be described hereinafter.

On top of the base layer 107 a back surface field (“BSF”) layer 108 isdeposited and used to reduce recombination loss, preferably p+ AlGaInP.

The BSF layer 108 drives minority carriers from the region near thebase/BSF interface surface to minimize the effect of recombination loss.In other words, a BSF layer 18 reduces recombination loss at thebackside of the solar subcell A and thereby reduces the recombination inthe base.

On top of the BSF layer 108 is deposited a sequence of heavily dopedp-type and n-type layers 109 which forms a tunnel diode which is anohmic circuit element to connect subcell A to subcell B. These layersare preferably composed of p++ AlGaAs, and n++ InGaP.

On top of the tunnel diode layers 109 a window layer 110 is deposited,preferably n+InAlP. The window layer 110 used in the subcell B operatesto reduce the interface recombination loss. It should be apparent to oneskilled in the art, that additional layer(s) may be added or deleted inthe cell structure without departing from the scope of the presentinvention.

On top of the window layer 110 the layers of subcell B are deposited:the n-type emitter layer 111 and the p-type base layer 112. These layersare preferably composed of InGaP and In_(0.015)GaAs respectively (for aGe substrate or growth template), or InGaP and GaAs respectively (for aGaAs substrate), although any other suitable materials consistent withlattice constant and bandgap requirements may be used as well. Thus,subcell B may be composed of a GaAs, GaInP, GaInAs, GaAsSb, or GaInAsNemitter region and a GaAs, GaInAs, GaAsSb, or GaInAsN base region. Thedoping profile of layers 111 and 112 according to the present inventionwill be discussed in conjunction with FIG. 16.

In the preferred embodiment of the present invention, the middle subcellemitter has a band gap equal to the top subcell emitter, and the bottomsubcell emitter has a band gap greater than the band gap of the base ofthe middle subcell. Therefore, after fabrication of the solar cell, andimplementation and operation, neither the middle subcell B nor thebottom subcell C emitters will be exposed to absorbable radiation.Substantially radiation will be absorbed in the bases of cells B and C,which have narrower band gaps then the emitters. Therefore, theadvantages of using heterojunction subcells are: 1) the short wavelengthresponse for both subcells will improve, and 2) the bulk of theradiation is more effectively absorbed and collected in the narrowerband gap base. The effect will be to increase J_(sc).

On top of the cell B is deposited a BSF layer 113 which performs thesame function as the BSF layer 109. A p++/n++ tunnel diode 114 isdeposited over the BSF layer 113 similar to the layers 109, againforming an ohmic circuit element to connect subcell B to subcell C.These layers 114 are preferably compound of p++ AlGaAs and n++ InGaP.

A barrier layer 115, preferably composed of n-type InGa(Al)P, isdeposited over the tunnel diode 114, to a thickness of about 1.0 micron.Such bather layer is intended to prevent threading dislocations frompropagating, either opposite to the direction of growth into the middleand top subcells B and C, or in the direction of growth into the bottomsubcell A, and is more particularly described in copending U.S. patentapplication Ser. No. 11/860,183, filed Sep. 24, 2007.

A metamorphic layer (or graded interlayer) 116 is deposited over thebarrier layer 115 using a surfactant. Layer 116 is preferably acompositionally step-graded series of InGaAlAs layers, preferably withmonotonically changing lattice constant, so as to achieve a gradualtransition in lattice constant in the semiconductor structure fromsubcell B to subcell C while minimizing threading dislocations fromoccurring. The bandgap of layer 116 is constant throughout its thicknesspreferably approximately 1.5 eV or otherwise consistent with a valueslightly greater than the bandgap of the middle subcell B. The preferredembodiment of the graded interlayer may also be expressed as beingcomposed of (In_(x)Ga_(1-x))_(y)Al_(1-y)As, with x and y selected suchthat the band gap of the interlayer remains constant at approximately1.50 eV.

In the surfactant assisted growth of the metamorphic layer 116, asuitable chemical element is introduced into the reactor during thegrowth of layer 116 to improve the surface characteristics of the layer.In the preferred embodiment, such element may be a dopant or donor atomsuch as selenium (Se) or tellurium (Te). Small amounts of Se or Te aretherefore incorporated in the metamorphic layer 116, and remain in thefinished solar cell. Although Se or Te are the preferred n-type dopantatoms, other non-isoelectronic surfactants may be used as well.

Surfactant assisted growth results in a much smoother or planarizedsurface. Since the surface topography affects the bulk properties of thesemiconductor material as it grows and the layer becomes thicker, theuse of the surfactants minimizes threading dislocations in the activeregions, and therefore improves overall solar cell efficiency.

As an alternative to the use of non-isoelectronic one may use anisoelectronic surfactant. The term “isoelectronic” refers to surfactantssuch as antimony (Sb) or bismuth (Bi), since such elements have the samenumber of valence electrons as the P of InGaP, or as in InGaAlAs, in themetamorphic buffer layer. Such Sb or Bi surfactants will not typicallybe incorporated into the metamorphic layer 116.

In an alternative embodiment where the solar cell has only two subcells,and the “middle” cell B is the uppermost or top subcell in the finalsolar cell, wherein the “top” subcell B would typically have a bandgapof 1.8 to 1.9 eV, then the band gap of the interlayer would remainconstant at 1.9 eV.

In the inverted metamorphic structure described in the Wanlass et al.paper cited above, the metamorphic layer consists of ninecompositionally graded InGaP steps, with each step layer having athickness of 0.25 micron. As a result, each layer of Wanlass et al. hasa different bandgap. In the preferred embodiment of the presentinvention, the layer 116 is composed of a plurality of layers ofInGaAlAs, with monotonically changing lattice constant, each layerhaving the same bandgap, approximately 1.5 eV.

The advantage of utilizing a constant bandgap material such as InGaAlAsis that arsenide-based semiconductor material is much easier to processin standard commercial MOCVD reactors, while the small amount ofaluminum assures radiation transparency of the metamorphic layers.

Although the preferred embodiment of the present invention utilizes aplurality of layers of InGaAlAs for the metamorphic layer 116 forreasons of manufacturability and radiation transparency, otherembodiments of the present invention may utilize different materialsystems to achieve a change in lattice constant from subcell B tosubcell C. Thus, the system of Wanlass using compositionally gradedInGaP is a second embodiment of the present invention. Other embodimentsof the present invention may utilize continuously graded, as opposed tostep graded, materials. More generally, the graded interlayer may becomposed of any of the As, P, N, Sb based III-V compound semiconductorssubject to the constraints of having the in-plane lattice parametergreater or equal to that of the second solar cell and less than or equalto that of the third solar cell, and having a bandgap energy greaterthan that of the second solar cell.

In another embodiment of the present invention, an optional secondbarrier layer 117 may be deposited over the InGaAlAs metamorphic layer116. The second bather layer 117 will typically have a differentcomposition than that of bather layer 115, and performs essentially thesame function of preventing threading dislocations from propagating. Inthe preferred embodiment, barrier layer 117 is n+ type GaInP.

A window layer 118 preferably composed of n+ type GaInP is thendeposited over the barrier layer 117 (or directly over layer 116, in theabsence of a second barrier layer). This window layer operates to reducethe recombination loss in subcell “C”. It should be apparent to oneskilled in the art that additional layers may be added or deleted in thecell structure without departing from the scope of the presentinvention.

On top of the window layer 118, the layers of cell C are deposited: then+ emitter layer 119, and the p-type base layer 120. These layers arepreferably composed of n type InGaAs and p type InGaAs respectively, orn type InGaP and p type InGaAs for a heterojunction subcell, althoughanother suitable materials consistent with lattice constant and bandgaprequirements may be used as well. The doping profile of layers 119 and120 will be discussed in connection with FIG. 16.

A BSF layer 121, preferably composed of InGaAlAs, is then deposited ontop of the cell C, the BSF layer performing the same function as the BSFlayers 108 and 113.

Finally a high band gap contact layer 122, preferably composed ofInGaAlAs, is deposited on the BSF layer 121.

This contact layer added to the bottom (non-illuminated) side of a lowerband gap photovoltaic cell, in a single or a multijunction photovoltaiccell, can be formulated to reduce absorption of the light that passesthrough the cell, so that (1) an ohmic metal contact layer below(non-illuminated side) it will also act as a mirror layer, and (2) thecontact layer doesn't have to be selectively etched off, to preventabsorption.

It should be apparent to one skilled in the art, that additionallayer(s) may be added or deleted in the cell structure without departingfrom the scope of the present invention.

FIG. 3 is a cross-sectional view of the solar cell of FIG. 2 after thenext process step in which a metal contact layer 123 is deposited overthe p+ semiconductor contact layer 122. The metal is preferably thesequence of metal layers Ti/Au/Ag/Au.

Also, the metal contact scheme chosen is one that has a planar interfacewith the semiconductor, after heat treatment to activate the ohmiccontact. This is done so that (1) a dielectric layer separating themetal from the semiconductor doesn't have to be deposited andselectively etched in the metal contact areas; and (2) the contact layeris specularly reflective over the wavelength range of interest.

FIG. 4 is a cross-sectional view of the solar cell of FIG. 3 after thenext process step in which an adhesive layer 124 is deposited over themetal layer 123. The adhesive is preferably Wafer Bond (manufactured byBrewer Science, Inc. of Rolla, Mo.).

FIG. 5A is a cross-sectional view of the solar cell of FIG. 4 after thenext process step in which a surrogate substrate 125, preferablysapphire, is attached. Alternative, the surrogate substrate may be GaAs,Ge or Si, or other suitable material. The surrogate substrate is about40 mils in thickness, and is perforated with holes about 1 mm indiameter, spaced 4 mm apart, to aid in subsequent removal of theadhesive and the substrate. As an alternative to using an adhesive layer124, a suitable substrate (e.g., GaAs) may be permanently bonded to themetal layer 123.

FIG. 5B is a cross-sectional view of the solar cell of FIG. 5A after thenext process step in which the original substrate is removed by asequence of lapping and/or etching steps in which the substrate 101, andthe buffer layer 103 are removed. The choice of a particular etchant isgrowth substrate dependent.

FIG. 5C is a cross-sectional view of the solar cell of FIG. 5B with theorientation with the surrogate substrate 125 being at the bottom of theFigure. Subsequent Figures in this application will assume suchorientation.

FIG. 6 is a simplified cross-sectional view of the solar cell of FIG. 5Bdepicting just a few of the top layers and lower layers over thesurrogate substrate 125.

FIG. 7 is a cross-sectional view of the solar cell of FIG. 6 after thenext process step in which the etch stop layer 103 is removed by aHCl/H₂O solution.

FIG. 8 is a cross-sectional view of the solar cell of FIG. 7 after thenext sequence of process steps in which a photoresist mask (not shown)is placed over the contact layer 104 to form the grid lines 501. As willbe described in greater detail below, the grid lines 501 are depositedvia evaporation and lithographically patterned and deposited over thecontact layer 104. The mask is subsequently lifted off to form thefinished metal grid lines 501 as depicted in the Figures.

As noted above, the present invention is directed to the composition ofthe metal contact. One composition considered was the sequence of layersAu/Ge/Pd/Au, and another was Pd/Ge/Ti/Pd/Au.

Initial investigation of both compositions was done at 280° C. The Au/Gebased contact required a 60 minute anneal (45 minute anneal resulted ina rectifying contact) to give a specific contact resistance of 3×10⁻⁶Ω-cm².

The Pd/Ge based contact sintered for 5 min at 280° C. yielded anacceptable specific contact resistance of nominally 1×10⁻⁴ Ω-cm². Themore reasonable sintering time for the required specific contactresistance (≧1×10⁻³ Ω-cm²) led to the identification of the Pd/Ge basedmetallization as optimum for the fabrication process of the solar celldescribed herein. Moreover, the adhesive thermal property permittedlowering the sinter temperature to 205° C. At this low sintertemperature, a time of 35 minutes was required to achieve a specificcontact resistance of 1×10⁻⁴ Ω-cm².

A more detailed description of the preferred embodiment is as follows.Prior to the metal deposition of the n-contact, residual oxide isremoved from the wafer by soaking the wafer in a solution of 15H₂O)1NH₄OH for one minute, and spin dried in N₂. The wafer is then loaded inthe deposition chamber within 30 minutes to preclude excessive oxidegrowth. The metallization in the preferred embodiment, i.e. the sequenceof layers of 50 nm Pd/100 nm Ge/30 nm Ti/30 nm Pd/5 μm Ag/100 nm Au, ise-beam evaporated during one vacuum cycle. The background chamberpressure at the beginning of deposition is 5×10⁻⁷ torr. Followingdeposition, the grid lines 501 and bus bar are defined by liftoff. Thecontact sintering is then performed in the lab oratory ambientatmosphere on a hot plate. The wafer is placed grid side down on a cleansilicon wafer on a hot plate, set at a temperature of 120° C. The waferand silicon carrier are allowed to equilibrate for 5 min. The hot plateis then set at the sintering temperature (e.g. a set point of 215). Tenminutes are allowed for the wafer to attain the sintering temperature.The metal contact then sinters for 40 min. The hot plate temperature isthen dropped in ten minutes to 120° C. The Si carrier and wafer areremoved from the hot plate. Transmission line method (TLM) patternsformed on the solar cell wafer permit specific contact resistancemeasurements, as will be subsequently described in connection with FIG.17.

A variety of different Pd/Ge based contacts are suitable for applicationin the present invention, including, but not limited to Pd/Ge/Au,Pd/Ge/Ag, Pd/Ge/Pt/Au, Pd/Ge/Pt/Ag, dried Pd/Ge/Pt/Ag/Au. Those skilledin the art would be able to select the most suitable combination for thesemiconductor layers and fabrication processes being utilized.

Two embodiments of the contact composition according to the presentinvention, viz. Au/Ge/Pd/Au or Pd/Ge/Ti/Pd/Au, are illustrated in FIG.8. The preferred embodiment is Pd/Ge/Ti/Pd/Au, while other Pd/Ge basedlayer sequences provide substantially similar results and are within thescope of the present invention.

FIG. 9 is a cross-sectional view of the solar cell of FIG. 8 after thenext process step in which the grid lines are used as a mask to etchdown the surface to the window layer 105 using a citric acid/peroxideetching mixture.

FIG. 10A is a top plan view of a wafer in which four solar cells areimplemented. The depiction of four cells is for illustration forpurposes only, and the present invention is not limited to any specificnumber of cells per wafer.

In each cell there are grid lines 501 (more particularly shown incross-section in FIG. 9), an interconnecting bus line 502, and a contactpad 503. The geometry and number of grid and bus lines is illustrativeand the present invention is not limited to the illustrated embodiment.

FIG. 10B is a bottom plan view of the wafer with four solar cells shownin FIG. 10A.

FIG. 11 is a cross-sectional view of the solar cell of FIG. 11 after thenext process step in which an antireflective (ARC) dielectric coatinglayer 130 is applied over the entire surface of the “bottom” side of thewafer with the grid lines 501.

FIG. 12 is a cross-sectional view of the solar cell of FIG. 11 after thenext process step according to the present invention in which a channel510 or portion of the semiconductor structure is etched down to themetal layer 123 using phosphide and arsenide etchants defining aperipheral boundary and leaving a mesa structure which constitutes thesolar cell. The cross-section depicted in FIG. 12 is that as seen fromthe A-A plane shown in FIG. 13.

FIG. 13 is a top plan view of the wafer of FIG. 12 depicting the channel510 etched around the periphery of each cell using phosphide andarsenide etchants.

FIG. 14A is a cross-sectional view of the solar cell of FIG. 12 afterthe next process step in a first embodiment of the present invention inwhich the surrogate substrate 125 is appropriately thinned to arelatively thin layer 125 a, by grinding, lapping, or etching.

FIG. 14B is a cross-sectional view of the solar cell of FIG. 14A afterthe next process step in a second embodiment of the present invention inwhich a cover glass is secured to the top of the cell by an adhesive.

FIG. 15 is a cross-sectional view of the solar cell of FIG. 14B afterthe next process step in a third embodiment of the present invention inwhich a cover glass is secured to the top of the cell and the surrogatesubstrate 125 is entirely removed, leaving only the metal contact layer123 which forms the backside contact of the solar cell. The surrogatesubstrate may be reused in subsequent wafer processing operations.

FIG. 16 is a graph of a doping profile in the emitter and base layers inone or more subcells of the inverted metamorphic multijunction solarcell of the present invention. The various doping profiles within thescope of the present invention, and the advantages of such dopingprofiles are more particularly described in copending U.S. patentapplication Ser. No. 11/956,069 filed Dec. 13, 2007, herein incorporatedby reference. The doping profiles depicted herein are merelyillustrative, and other more complex profiles may be utilized as wouldbe apparent to those skilled in the art without departing from the scopeof the present invention.

FIG. 17 is a graph depicting the results of a TLM pattern measurement onwafers with contacts 501 formed with Pd/Ge/Ti/Pd/Au according to thepresent invention, with the resistance between electrodes (measured inohms) shown as a function of the electrode separation, depicted alongthe x-axis.

It will be understood that each of the elements described above, or twoor more together, also may find a useful application in other types ofconstructions differing from the types of constructions described above.

Although the preferred embodiment of the present invention utilizes avertical stack of three subcells, the present invention can apply tostacks with fewer or greater number of subcells, i.e. two junctioncells, four junction cells, five junction cells, etc. In the case offour or more junction cells, the use of more than one metamorphicgrading interlayer may also be utilized.

In addition, although the present embodiment is configured with top andbottom electrical contacts, the subcells may alternatively be contactedby means of metal contacts to laterally conductive semiconductor layersbetween the subcells. Such arrangements may be used to form 3-terminal,4-terminal, and in general, n-terminal devices. The subcells can beinterconnected in circuits using these additional terminals such thatmost of the available photogenerated current density in each subcell canbe used effectively, leading to high efficiency for the multijunctioncell, notwithstanding that the photogenerated current densities aretypically different in the various subcells.

As noted above, the present invention may utilize an arrangement of oneor more, or all, homojunction cells or subcells, i.e., a cell or subcellin which the p-n junction is formed between a p-type semiconductor andan n-type semiconductor both of which have the same chemical compositionand the same band gap, differing only in the dopant species and types,and one or more heterojunction cells or subcells. Subcell A, with p-typeand n-type InGaP is one example of a homojunction subcell.Alternatively, as more particularly described in U.S. patent applicationSer. No. 12/023,772 filed Jan. 31, 2008, the present invention mayutilize one or more, or all, heterojunction cells or subcells, i.e., acell or subcell in which the p-n junction is formed between a p-typesemiconductor and an n-type semiconductor having different chemicalcompositions of the semiconductor material in the n-type regions, and/ordifferent band gap energies in the p-type regions, in addition toutilizing different dopant species and type in the p-type and n-typeregions that form the p-n junction.

The composition of the window or BSF layers may utilize othersemiconductor compounds, subject to lattice constant and band gaprequirements, and may include AlInP, AlAs, AlP, AlGaInP, AlGaAsP,AlGaInAs, AlGaInPAs, GaInP, GaInAs, GaInPAs, AlGaAs, AlInAs, AlInPAs,GaAsSb, AlAsSb, GaAlAsSb, AlInSb, GaInSb, AlGaInSb, AlN, GaN, InN,GaInN, AlGaInN, GaInNAs, AlGaInNAs, ZnSSe, CdSSe, and similar materials,and still fall within the spirit of the present invention.

While the invention has been illustrated and described as embodied in ainverted metamorphic multijunction solar cell, it is not intended to belimited to the details shown, since various modifications and structuralchanges may be made without departing in any way from the spirit of thepresent invention.

Thus, while the description of this invention has focused primarily onsolar cells or photovoltaic devices, persons skilled in the art knowthat other electronic and optoelectronic devices, such as, transistors,thermophotovoltaic (TPV) cells, photodetectors and light-emitting diodes(LEDS) are very similar in structure, physics, and materials tophotovoltaic devices with some minor variations in doping and theminority carrier lifetime. For example, photodetectors can be the samematerials and structures as the photovoltaic devices described above,but perhaps more lightly-doped for sensitivity rather than powerproduction. On the other hand LEDs and also be made with similarstructures and materials, but perhaps more heavily-doped to shortenrecombination time, thus radiative lifetime to produce light instead ofpower. Therefore, this invention also applies to photodetectors and LEDswith structures, compositions of matter, articles of manufacture, andimprovements as described above for photovoltaic cells.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

The invention claimed is:
 1. A method of forming a multijunction solarcell comprising a top subcell, at least one middle subcell, and a bottomsubcell, the method comprising: forming a semiconductor contact layercomposed of GaAs over the top subcell; depositing a metal contact layerincluding a germanium layer and a palladium layer over the semiconductorcontact layer by e-beam evaporation; and forming an ohmic metal contactfrom the metal contact layer to the semiconductor contact layer having aspecific contact resistance of less than 2×10⁻⁴ ohms-cm², wherein themetal contact layer comprises a sequence of layers selected from of thegroup consisting of Pd/Ge/Au, Pd/Ge/Ag, Pd/Ge/Pt/Au, Pd/Ge/Pt/Ag,Pd/Ge/Pt/Ag/Au, Au/Ge/Pd/Au, Pd/Ge/Ti/Pd/Au, and Pd/Ge/Ti/Pd/Ag/Au;, andwherein the metal contact layer is heated at 210 degrees C. or less. 2.The method as defined in claim 1, wherein the palladium layer has athickness of 50 nm and the germanium layer has a thickness of 100 nm. 3.The method as defined in claim 1, wherein the palladium layer isdisposed adjacent to the semiconductor contact layer, and the germaniumlayer is disposed on top of the palladium layer.
 4. The method asdefined in claim 1, wherein the metal contact layer is heated forapproximately 35 minutes or less.
 5. The method as defined in claim 1,wherein the specific contact resistance between the metal contact layerand the semiconductor contact layer is less than 5×10⁻⁶ ohms-cm².
 6. Themethod as defined in claim 1, wherein the specific contact resistance isless than 1×10⁻⁴ohms-cm².
 7. The method as defined in claim 1, whereinthe heating is performed in ambient atmosphere on a hot plate.
 8. Themethod as defined in claim 1, wherein the multijunction solar cell is atriple junction solar cell composed of III-V compound semiconductormaterial including the top subcell, the at least one middle subcell, andthe bottom subcell, wherein the top subcell has a first band gap; the atleast one middle subcell has a second band gap smaller than said firstband gap; a graded interlayer is disposed over said at least one middlesubcell that is compositionally graded to lattice match the at least onemiddle subcell on one side and the bottom subcell on the other side; thebottom subcell disposed over said graded interlayer has a fourth bandgap smaller than said second band gap such that said bottom subcell islattice mismatched with respect to said at least one middle subcell. 9.The method as defined in claim 8, wherein the upper subcell is composedof InGa(Al)P.
 10. The method as defined in claim 8, wherein the at leastone middle subcell is composed of an GaAs, GaInP, GaInAs, GaAsSb, orGaInAsN emitter region and a GaAs, GaInAs, GaAsSb, or GaInAsN baseregion.
 11. The method as defined in claim 8, wherein the lower subcellis composed of an InGaAs base and emitter layer, or a InGaAs base layerand a InGaP emitter layer.
 12. The method as defined in claim 8, whereinthe graded interlayer is composed of InGaAlAs.
 13. The method as definedin claim 8, wherein the graded interlayer has a band gap ofapproximately 1.5 eV throughout its thickness.
 14. The method as definedin claim 8, wherein the graded interlayer is composed of any of the As,N, Sb based III-V compound semiconductors subject to a constraint ofhaving an in-plane lattice parameter greater or equal to that of thesecond solar subcell and less than or equal to that of the second solarsubcell and less than or equal to that of the third solar subcell, andhaving a band gap energy greater than that of the second solar subcell.15. The method as defined in claim 8, wherein the graded interlayer iscomposed of (In_(x)Ga_(1-x))_(y)Al_(1-y)As, with x and y selected suchthat the band gap of the transition material remains constant throughoutits thickness.
 16. The method as defined in claim 15, wherein saidgraded interlayer is composed of nine or more steps of layers ofsemiconductor material with monotonically changing lattice constant andconstant band gap.
 17. A method of forming a multijunction solar cellcomprising a top subcell, at least one middle subcell, and a bottomsubcell, the method comprising: forming a semiconductor contact layercomposed of GaAs over the top subcell; depositing a metal contact layerincluding a germanium layer and a palladium layer over the semiconductorcontact layer; and forming an ohmic metal contact from the metal contactlayer to the semiconductor contact layer having a specific contactresistance of less than 2×10⁻⁴ohms-cm², wherein the metal contact layercomprises a sequence of layers selected from of the group consisting ofPd/Ge/Au, Pd/Ge/Ag, Pd/Ge/Pt/Au, Pd/Ge/Pt/Ag, Pd/Ge/Pt/Ag/Au,Au/Ge/Pd/Au, Pd/Ge/Ti/Pd/Au, and Pd/Ge/Ti/Pd/Ag/Au;, and wherein themetal contact layer is heated on a hot plate in ambient atmosphere. 18.The method as defined in claim 17, wherein the palladium layer isdisposed adjacent to the semiconductor contact layer, and the germaniumlayer is disposed on top of the palladium layer.
 19. The method asdefined in claim 17, wherein the specific contact resistance between themetal contact layer and the semiconductor contact layer is less than5×10⁻⁶ ohms-cm².
 20. The method as defined in claim 17, wherein thespecific contact resistance is less than 1×10⁻⁴ ohms-cm².